In such case, you need to restore GRUB boot manager. Follow these steps carefully and restore your old GRUB:

Boot through Ubuntu Linux Live CD/DVD

Start terminal and give following command:

$ sudo grub

You will then enter in grub configuration mode. Here, give following command:

$ root (hd0,0)

Then give following command:

$ setup (hd%1,%2) - here, %1,%2 is the disk volume and partition value of your original Linux installation where /boot/grub is already installed. For example, in my machine, it is on disk 1, partition 6, so the value will be – (hd0,5). So, be careful here else your Grub will not be restored, and you will get error message instead.

Finally, give following command within grub configuration mode:

$ quit

$ exit

Now, reboot (do not forget to remove Linux Live CD/DVD to boot from HDD). If everything had gone right, your Grub boot manager will load and prompt you to choose to boot from either Linux or Windows 7. Enjoy!